SunEdison Inc (SUNE.N), once the fastest-growing U.S. renewable energy company, filed for Chapter 11 bankruptcy protection on Thursday after a short-lived but aggressive binge of debt-fueled acquisitions proved unsustainable.

In its bankruptcy filing, the company said it had assets of $20.7 billion and liabilities of $16.1 billion as of Sept. 30.

SunEdison's two publicly traded subsidiaries, TerraForm Power Inc (TERP.O) and TerraForm Global Inc (GLBL.O), are not part of the bankruptcy. In a statement, the companies, known as yieldcos, said they had sufficient liquidity to operate and that their assets are not available to satisfy the claims of SunEdison creditors.

The bankruptcy "will present challenges," however, including with financing agreements for certain projects, the yieldcos said.

Chatila was named CEO of what was then called MEMC Electronic Materials in 2009 and almost immediately bought fledgling solar project developer SunEdison. The company changed its name four years later and embarked on a rapid expansion that included entering new businesses like wind and energy storage and taking on projects worldwide. That growth racked up billions of dollars of debt.

Solar industry watchers said the bankruptcy was not a reflection of the sector, which is growing rapidly.

"SunEdison had a balance sheet that is way out of line with any other solar company," said Shayle Kann, senior vice president and renewable energy research firm GTM Research. "The projects themselves are good. They just bought too much to quickly."

The company said it secured up to $300 million in new financing from its first-lien and second-lien lenders, which is subject to court approval. The money will be used to support SunEdison's operations during its bankruptcy.

"Our decision to initiate a court-supervised restructuring was a difficult but important step to address our immediate liquidity issues," said Ahmad Chatila, SunEdison chief executive officer.

He said the company planned to use Chapter 11 to reduce debt, shed non-core operations and take steps to get the most value out of its technology and intellectual property.

Shares of SunEdison were halted, and last traded at about 34 cents on the New York Stock Exchange. The company's stock traded as high as $33.44 in July 2015. Shares of First Solar Inc (FSLR.O) were up slightly and shares of SolarCity Corp (SCTY.O) were up 2.9 percent in late trading. An index of solar shares .SUNIDX gained 0.5 percent.

Major SunEdison shareholders include OppenheimerFunds Inc with a 11.9 percent stake, BlackRock Inc (BLK.N) with a 6.5 percent stake, The Vanguard Group with a 6.4 percent stake and Adage Capital Partners GP LLC with a 5.4 percent stake, according to court documents.

Hedge fund Greenlight Capital, led by David Einhorn, earlier this week reported that it had sharply cut its stake in SunEdison. Greenlight, which won a board seat at the company earlier this year, declined to comment on the bankruptcy.

Investors began to lose confidence in SunEdison's supercharged expansion last summer, when the company announced a $2.2 billion deal to acquire rooftop solar installer Vivint Solar Inc (VSLR.N). At the time, renewable energy stocks were under pressure, in part because falling oil prices sparked concerns about demand for alternative energy sources.

The Vivint deal touched off litigation involving SunEdison's yieldcos, the listed subsidiaries that own and operate renewable energy assets, many of which were acquired from SunEdison.